The invention relates to calibrating test equipment.
Automatic test equipment (ATE) is quite often used to rapidly evaluate the performance of electronic devices (e.g., semiconductor memory devices) to determine if the devices meet predetermined specifications (timing specifications, for example). For example, for each device, the equipment might test the device to obtain a measured specification value (a time, for example) that represents how fast the device performs an operation.
The test equipment may obtain a value that differs from a correct, actual specification value for the device. This difference may be caused by one of many possible sources of variation, such as, for example, variations introduced by test patterns and tester channels that are used to test the device.
Referring to FIG. 1, as a result of these variations, the specification values measured by one or more pieces of test equipment may be represented by a Gaussian distribution 10. Typically, the average value of the distribution 10 is very close to the actual specification value. In light of the distribution 10, there is a beta risk and an alpha risk for the testing.
Assuming that the measured specification value is determinative of whether the device passes a quality control test, the beta risk represents the probability that a bad device passes the test. As an example, when a minimum response time (a time t.sub.AA of a memory device, for example) is being measured, the beta risk is the portion of the distribution 10 to the left of the average time. Thus, although the device being tested may have an actual time near the average time, the test results may show the device has a faster time. To minimize the beta risk, a guard band threshold 14 is set to reject all times falling on the side of the threshold 14 that is farthest from the average time.
The alpha risk represents the probability that a good device fails the test. As an example, when a minimum response time of the part (e.g., t.sub.AA) is being measured, the alpha risk is the portion of the distribution 10 to the right of the average time. Thus, although the device being tested may have an actual time near the average time, the test results may show the device has a slower time.
To test a device, typically different channels of the test equipment are connected to different pins of the device. The test equipment uses the channels to drive some of the pins as inputs and monitor some of the pins as outputs. Each channel typically needs to be routinely calibrated. Otherwise, the measured specification values may vary substantially from the actual specification values.